The present invention is generally directed to a spindle position/speed control unit which utilizes a hollow position/speed detector attachable to the spindle, and more particularly, to a spindle position/speed control unit capable of simplifying the mounting structure of the detector and controlling both the position and speed of the spindle with high accuracy. The invention finds wide applicability in the field of numerically controlled machine tools, notably lathes.
FIG. 3 is a block diagram illustrating the driving system used for the main spindle of a C-axis type machine tool equipped with a conventional numerical control unit. Referring to FIG. 3, numeral 1 designates a numerical control unit; 2, a main spindle drive control unit; 3, an induction motor; 4, a speed detector; 5, a low resolution position detector; 6, a high resolution position detector; 7, the main spindle; 8, a connection gear for the induction motor 3; 9, a connection gear for the position detector 5; 10, a connection gear for the position detector 6; 51, a speed detecting circuit; 54, a low resolution position detecting circuit; and 57, a high resolution position detecting circuit.
Referring again to FIG. 3, a speed command wr* issued from the numerical control unit 1 is output in the form of a 3-phase AC current command via the main spindle drive control unit 2 to the induction motor 3 that in turn rotates so as to follow the command wr*. To improve the follow-up (feedback) properties, a so-called closed-loop is formed, which formation involves the steps of detecting the speed of the induction motor 3 by inputting the output waveforms A of the speed detector 4 into the speed detecting circuit 51 provided in the main spindle drive control unit 2, and feeding back the detected value in the form of wr.
Rotation of the induction motor 3 is transferred via the connection gear 8 to the main spindle 7, thus driving the main spindle 7. The gear ratio of the connection gear is determined depending on the application.
A positional command .theta.r* alternatively issued from the numerical control unit 1 is output as a 3-phase AC current command via the main spindle drive control unit 2 to the induction motor 3, which in turn rotates so as to follow the positional command .theta.r*. In this case, for the purpose of improving the positional follow-up properties, a low resolution position closed-loop feed back system is set up, this involving the steps of detecting the position of the main spindle 7 by inputting output waveforms B from the low resolution position detector 5 into a low resolution position detecting circuit 54 incorporated into the main spindle drive control unit 2, and feeding back the detected value in the form of .theta.r.sub.1.
The position of the main spindle 7 can also be detected by inputting the output waveforms C of the high resolution position detector 6 into a high resolution position detecting circuit 57 provided inside the main spindle drive control unit 2. The detected value is fed back in the form of .theta.r.sub.2, thus setting up a high resolution position closed-loop.
The operation of this system will now be described. When performing ordinary lathe operations using the main spindle 7, the numerical control unit 1 outputs a speed command wr* corresponding to the desired speed of the main spindle 7, while the main spindle drive control unit 2 operates to make the actual speed wr of the induction motor 3 follow the speed command wr*, the speed wr being detected by the speed detecting circuit 51.
In the case of effecting a C-axis operation (for example, drilling a hole in the workpiece parallel to the rotational axis of the workpiece, or forming a contour on a face of the workpiece), the numerical control unit 1 outputs a positional command .theta.r* corresponding to the desired position of the main spindle 7, while the main spindle drive control unit 2 operates to make the position .theta.r.sub.2 of the main spindle 7 follow the positional command .theta.r*, the position .theta.r.sub.2 being detected by the high resolution position detecting circuit 57. The C-axis operation thus involves the use of the high resolution detector 6, which has a resolution of about 360,000-pulses/revolution, because positional accuracy as high as 1/1000 degree may be required at the end of the main spindle for these machining operations.
Next, the operation of the position detector 5 will be described. The position detector 5 has a wide variety of applications, as follows:
1. Where the main spindle 7 is brought into an oriented stop operation for the purpose of positioning the workpiece for machining based on mechanical fixing by the insertion of knock pins or the like, the low resolution position detecting circuit 54 detects the position detection value .theta.r.sub.1 to form a positional loop, thereby stopping the main spindle 7 at the desired position.
2. When the speed of the main spindle 7 is to be displayed on the CRT of the numerical control unit 1, the output waveforms B of the positional detector 5 are input to the numerical control unit 1. The speed of the main spindle is calculated from the waveform variations per unit time in the numerical control unit, and the resulting speed value is displayed on the CRT.
3. When carrying out synchronous operations with other elements or tools, which may operate along other axes, such as in the case of screw cutting (synchronizing rotation with X-axis and Z-axis movements), polygonal machining (synchronizing with a rotary tool spindle), or workpiece transfer or protrusion cutting (synchronizing positionally with a second opposed main spindle of the lathe), which require synchronization between the position of the main spindle 7 and other shafts or spindles, the positional detection value .theta.r.sub.1 is detected by means of the low resolution position detecting circuit 54. A positional loop is formed so that the detected value .theta.r.sub.1 follows the main spindle positional command .theta.r* transmitted from the numerical control unit 1, to position-synchronize the main spindle with such other spindles. The position detector 5 typically has a resolution of approximately 1024-4096-pulses/revolution.
Referring again to FIG. 3, in the past there have been employed only a pair of simply constructed connection gears 8 for connecting the induction motor 3 to the main spindle 7. As a matter of course, however, there may be plural pairs of such gears, for driving at differing ratios. For instance, the following three sets of gearing may be used.
______________________________________ L speed gears connection gear ratio = 10:1 H speed gears connection gear ratio = 1:1 C-axis operation gears connection gear ratio = 100:1 ______________________________________
It is to be noted that the connection gearing 9 and 10 may be replaced by belts.
As discussed above, when controlling the main spindle for various operations or when effecting the C-axis operation, one of the position detectors or the speed detector may be needed, depending on the respective operation. As a result, a plurality of detectors are mounted at a plurality of locations. For example, as illustrated in FIG. 3, the speed detector 4 is mounted so as to be connected directly to the induction motor 3, while the position detectors 5 and 6 are mounted via the connection gears 9 and 10 on the main spindle 7. (The position detectors 5 and 6 may be accommodated in the same package, as a result of which the single position detector and single connection gear suffice for the arrangement.)
The reason why the connection gears 9 and 10 are located as they are is as follows: The lathe generally provides a bar feeder function, and therefore the main spindle 7 is made hollow to permit the passage of the workpiece to be machined. The positional detectors must not hinder the passage of the workpiece, and hence it is impossible to attach them directly to the main spindle 7. For this reason, in great majority of cases the position detectors are mounted through connection gears 9 and 10 having a gear ratio of 1:1.